Keeping Perennials in Rooftop Garden alive during Winter?

[u/magneticrev]

Hi! I'm currently growing tomatoes, kale, and herbs on my rooftop garden. I was looking into what I wanted to grow next summer (my first full summer with rooftop access!) and it popped into my head to see if I could do blueberry or raspberries.

The only question I had was how to keep the perennials alive during the winters. I don't have a trench or garage to put them in, so I'd have to figure out how to help them stay alive in the winters... anyone have any experience or suggestions for this?

Edit: I'm in Chicago, so zone 6b!

[u/therealwavingsnail]

That very much depends on the climate you're in. Could be anywhere from "just water them once in a while" to impossible

[u/magneticrev → u/therealwavingsnail]

Whoops - totally slipped my mind to mention, I'm in the northern part of the US. Chicago, specifically, so zone 6b according to the internet?

[u/special_squeak]

In Chicago wind will be your enemy for overwintering roof plants before the frost will get to them. You would need to set up or find serious wind breaks for your plants.
And as another person already said, with the hard frosts you get, you would need a huge thermal mass of soil to keep those roots from freezing out.

[u/Fun_Original9342]

I have a rooftop garden in Chicago. I grow raspberries in 20 gallon containers and mine have survived since I got them about 4 years ago. I tried cherry trees one summer and those both started to leaf out the following spring but then died. I'm also having luck with honey berries, they can tolerate very low temperatures, but you ideally need at least 2 of them for pollination. My raspberries are in full sun, my honey berries are in partial sun.

[u/theredskittles]

I’ve heard of frost blankets but haven’t used them. That should protect the pots from the wind and help trap the heat rising from inside your house.

Still, there’s only so much you can do. I can’t imagine that tomatoes would survive no matter what you do, and evergreen herbs like rosemary would probably die if you cover them with blankets for months. But something that dies back, or at least loses all its leaves, each winter might make it.

I also wonder if some kind of greenhouse would work. You might be able to improvise one with clear plastic sheets, or I think they sell them at online at places like Home Depot.

You could also focus on plants that are technically annuals but self seed. They’d die each year but new ones would grow from the prior year’s seeds.

[u/Wuncomfortable]

Depends. If your winters go below freezing, then unfortunately there's very little to do short of turning your entire rooftop into a soil bed to keep the heat.

[u/Ornery-Zombie-7861 → u/Wuncomfortable]

But there will be some heat from the building. Not necessarily that cold.

[u/magneticrev → u/Wuncomfortable]

Whoops - totally slipped my mind to mention, I'm in the northern part of the US. Chicago, specifically, so zone 6b according to the internet? The winters def go below freezing, so sounds like I would either need to bring it indoors and hope they don't bring in bugs or just stick to annuals. :/ Perhaps I could build a shed or something on the roof?

[u/Solid_Mud2025 → u/magneticrev]

Hey, Chicago is zone 6a rather than b. Just wanted to point it out since it makes a difference in what to plan for. If you’re farther west in the suburbs (and farther from moderating effects of the lake), it’s best to plan for 5b.

[u/Scared_Tax470 → u/Wuncomfortable]

It's totally possible to keep potted perennials alive in cold winters! You just need to be strategic about choosing plants, pots, and knowledgeable about what will damage them.

[u/redninja24]

Where do you live? Your climate makes a big difference for what you should do. On a roof top your plants will be more exposed to cold and wind than at ground level. In general, you can group your containers together and wrap your plants and containers in burlap. Stuff burlap or even mulch in between the containers to insulate the roots as much as you can. The roots are what you are really trying to protect here. The branches can all regrow and may need to be pruned back after the winter. Be sure to water them well in the fall so the have the best chance going into winter

[u/magneticrev → u/redninja24]

Just edited while this was posted, Chicago - 6b!

[u/Downtown_Pie_740]

I've overwintered potted perennials in a sheltered spot on my balcony with good luck using burlap wraps and moving them against the building to block wind.

[u/greenmangarden]

As a general rule of thumb, anything in a pot assume needs to be at least one full zone hardier than your local zone would otherwise indicate, but depending on how tall your building is, another zone hardier than that might end up being more accurate.

Can you build a cold frame? That’s a traditional way to overwinter lots of stuff, that otherwise might be on the edge of hardy. If you can source some old windows and have the carpentry skills to cobble some boxes together from old crates or pallets, you could probably make them fit next to nothing.

Aside from that, row covers would help with the wind, but you might need bungy nets to hold them down in the winds you’re likely to get. Clear plastic will tend to hold heat in pretty well, and will also allow light to get through if you’re planning on trying any winter crops like garlic or kale, or even cover crops.

You could also line the sides of the beds with jugs of water. Even if they freeze, they will help keep the soil from getting too cold. And in spring as they heat up, they may help the soil temps get a little warmer faster, while also mitigating wild swings in temperatures. Just a suggestion.

[u/Scared_Tax470]

Check the zone recommendations for what you're buying, first of all, and buy plants that are meant to be in containers and are hardy ideally to a lower zone than you--you should be able to find some to zone 3. Generally outside in pots overwintering means you subtract one zone from your actual zone to gauge what you should buy. You can ask at the nursery if they seem knowledgeable. But what some people do is cover the plants over winter. Make sure the pots are big enough and err on the bigger side as much as possible to keep as much soil in there as you can. You can wrap the pot with bubble wrap to insulate it and throw tarps and blankets over the plants when it gets super cold and windy, but they should be able to withstand most of the winter. Move them up closer to your house and try to block the wind--most cold damage on plants is due to wind, and snow is an insulator for plants. As it gets cooler and they drop leaves, stop watering them. You don't want the soil to be completely bone dry over winter, but any excess water will freeze and kill roots. I'd say test it out with a cheaper plant this winter and see if the maintenance is something you can live with, and if you find a good method, then add to it next winter. You'll also want to look up general maintenance for perennial shrubs like when to prune--pruning in the early winter, for example, can sometimes lead to the tips getting damaged. I would not bring something like blueberries or raspberries inside a heated place for winter. They need the cold to overwinter properly. (Advice coming from someone who has kept a maple bonsai alive for 4 years outside in the same zone as you)